The invention relates to a separator for an electrochemical energy storage system. The invention also relates to a method for the production of a separator. The invention furthermore relates to an electrochemical energy storage system.
A lithium-sulphur battery is known from DE 10 2010 018 731 A1 which comprises a separator which comprises a non-woven fleece made from polymer fibers.
Furthermore, a resilient battery separating medium is known from DE 697 09 171 T2 which comprises a resilient, air laid fleece made from randomly orientated, entangled glass microfibers, wherein at least one part of the microfibers in the air laid fleece is further entangled in connection with the collection of microfibers to form the air laid, fibrous fleece in order to increase the tensile strength of the air laid, fibrous fleece; wherein the air laid, fibrous fleece has a mass of between approximately 50 and approximately 450 grams per square meter; wherein the microfibers have an average diameter of between approximately 1.0 and 2.0 micrometers; and the air laid, fibrous fleece has first and second main surfaces for contact with the main surfaces of the electrode plates of a battery in which the air laid, fibrous fleece is incorporated. Here, the separating medium has a clamping force which enables the separating medium to maintain contact with the electrode plates when repeated compressing and expansion is experienced in a direction perpendicular to the main surfaces of the air laid, fibrous fleece when the electrode plates expand and contract during the operating period of the battery.
The object of the invention is to specify a separator for an electrochemical energy storage system which is improved compared to the prior art, an improved method for the production of such a separator and an improved electrochemical energy storage system.
A separator for an electrochemical storage system is able to be arranged between an anode and a cathode of the electrochemical storage system, wherein the separator is formed from a semipermeable carrier material. It is provided according to the invention that the carrier material is formed from a fiberglass fleece, wherein at least one surface side of the fiberglass fleece is modified in such a way that it is impermeable for an active material of the cathode. Here, the active material of the cathode comprises sulphur compounds in particular.
When discharging the battery, sulphur compounds may arise which are potentially not completely converted into elemental sulphur during the charging process. A diffusion of these sulphur compounds to the anode are prevented or at least reduced by means of the impermeability of the separator. The sulphur compounds can form a lithium-sulphide layer on the anode, which considerably reduce the capacity and thus the life span of the battery. In addition, the active material embedded in the cathode, in particular sulphur, is gradually reduced. The separator formed in such a way thus increases the life span of the electrochemical storage system.
The fiberglass fleece is expediently permeable for an active material of the anode, wherein the active material of the anode comprises metal ions. The charging and discharging processes of the electrochemical storage system are thus ensured since the metal ions can be transported through the separator from the anode to the cathode and vice versa.
An electrolyte is thus arranged in the fiberglass fleece which, in a preferred embodiment variant, is an organic, liquid electrolyte. Alternatively, the electrolytes can also be solid-state electrolytes. The electrolyte ensures an ion exchange between the cathode and the anode.
A method according to the invention is provided for the production of the separator described above, the method comprising the following steps:
a) homogeneous wetting with concentrated acid on the surface of the fiberglass fleece at least on one surface side,
b) thermal pressing of the fiberglass fleece wetted with acid,
c) drying the pressed fiberglass fleece at a predetermined temperature,
d) cleaning the dried fiberglass fleece with demineralized water,
e) drying the cleaned fiberglass fleece using a vacuum, and
f) pressing the dried fiberglass fleece and subsequent cooling of the pressed fiberglass fleece.
The method enables a surface modification of at least one surface side of the fiberglass fleece, such that it is impermeable on the modified surface side for the active material of the cathode, in particular for sulphur compounds such as polysulphides, but still permeable for the active material of the anode, for example lithium ions. This enables an improved performance of the electrochemical energy storage system compared to prior art since the cathodic active material still remains in the cathode and an accumulation of cathodic active material in the anode is avoided to the greatest extent possible.
According to a preferred exemplary embodiment, the fiberglass fleece is wetted with nitric acid according to step a). A semipermeable layer is formed on the wetted surface by wetting the fiberglass fleece with nitric acid, the layer being impermeable for sulphur compounds in particular.
In order to create a mechanically stable surface modification of the fiberglass fleece, the fiberglass fleece is pressed flat after wetting with acid according to step b) and after drying and cleaning according to step f). A permanent surface modification of the fiberglass fleece occurs through flattening, by means of which high performance and a long life span of the electrochemical energy storage system can be achieved.
The invention furthermore relates to an electrochemical energy storage system having at least one separator which is described above.
Exemplary embodiments of the invention are illustrated in greater detail below by means of drawings.